The nitric oxide-cGMP signal transduction system acts as a "brake" on cardiac function and metabolism and may be protective against excessive sympathetic tone in hypertrophy. In failure, the myocardial cGMP level is chronically elevated but its negative functional and metabolic effects are reduced due to a defect in its signaling pathway. The objective of this proposal is to determine the mechanism responsible for the maladaptation of the nitric oxide-cGMP system in heart failure and correct it. The hypothesis to be tested is that high cGMP levels in failure result in down-regulation of the cGMP dependent protein kinase and decreased phosphorylation of sarcoplasmic reticulum calcium ATPase and release channel. The specific aim is to determine and correct the defect of this signaling system by chronically lowering cGMP in failure and raising it in controls. Cardiac myocytes isolated from adult dogs with normal, hypertrophied (aortic stenosis) and failing (rapid pacing) hearts, as well as from transgenic mice (high/low basal cGMP) with and without aortic banding will be used to determine the defect in cGMP signaling and its effect on function (video-edge detection), calcium transients and O2 consumption. To assess the relative importance of these altered signaling processes, we will examine their effects on local work and O2 consumption in the intact heart. The in vivo studies will be conducted in anesthetized, open-chest dogs 6 months after induction of hypertrophy with or without failure and compared to controls. Regional myocardial work will be assessed from segment length (ultrasonic dimension crystals) and contractile force (miniature force gauges). O2 consumption of the same area will be determined from regional blood flow and regional O2 saturation of hemoglobin (microspectrophotometry). These physiological measurements will be combined with biochemical assays for cAMP and cGMP and the respective components of their signaling pathway. The ultimate goal is to determine if the cGMP signaling defect observed in failure is what initiates decompensation. Impact of the proposal: the understanding of the defect in the nitric oxide-cGMP signal transduction system will permit the development of novel treatment strategies for the secondary prevention of congestive heart failure in man.